There are three main modes of heat transfer: conduction, convection, and radiation. Conduction involves the direct contact and transfer of heat between particles of a material. Convection occurs through the movement of a fluid like a gas or liquid that transports heat as it moves. Radiation can occur through empty space and involves the emission and absorption of electromagnetic waves between surfaces at different temperatures. Fourier's law of heat conduction describes conduction as proportional to the temperature gradient through a material. Newton's law of cooling describes convection as proportional to the temperature difference between a body and its surroundings. Stefan-Boltzmann law describes radiation as proportional to the temperature of a radiating surface.